A multitude of today's mobile communications systems rely upon interleaving, channel coding and adaptive modulation to ensure data is transmitted more reliably. When data is transmitted over a channel in the presence of noise, errors will inevitably occur. Such errors may create several consecutive anomalous bits in a given bit string. Burst errors may occur when signal transmission power drops below a threshold level or may be induced by either thermal noise from receiver input circuits or by radiated electromagnetic noise picked up by a receiver's antenna.
Data may be transmitted with control bits that enable a channel decoder to correct a maximum number of anomalous bits per given bit string length. If a burst error occurs, and more than this maximum number of bits are altered, the bit string cannot be correctly decoded. For this reason, the bits of a number of independent bit strings may be interleaved. Interleaving is a process of rearranging the ordering of a sequence of binary symbols in a deterministic manner. In communications technology, data from multiple channels may be interleaved so as to minimize the chance that large portions of data from any one channel are lost or degraded due to burst errors. For example, a binary value of a given length, such as a byte, from each of N channels is encoded into an N-byte bit string for transmission across a radio frequency (RF) channel. Following reception, the N-byte bit string is decoded back into the individual data streams of the N channels. As such, a burst error during the RF transmission will only affect a correctable number of bits for the bit string of any given channel, so the decoder can decode the bit string correctly. Examples of the various types of interleaving include, diagonal interleaving, block Interleaving, inter-block interleaving, and convolution interleaving.
Additionally, if all possible outputs of a channel correspond uniquely to a source input, there is no possibility of detecting errors in the transmission. The goal of a given channel encoding method is to represent source information in a manner that minimizes the probability of error in decoding. To accomplish this goal, channel coding incorporates the use of redundancy. To detect and possibly correct errors, a channel codeword sequence must be longer than the source sequence it represents. A good channel code is designed so that if errors occur in transmission, the output can still be identified with the correct input. This is possible because although incorrect, the output is sufficiently similar to the input to be recognizable. Examples of the various types of channel coding common in the art include, Turbo coding, Viterbi coding, Reed-Solomon coding, Trellis coding, parity codes and block coding.
Radio transmission of information traditionally involves employing electromagnetic waves or radio waves as a carrier. Where the carrier is transmitted as a sequence of fully duplicated wave cycles or wavelets, no information is considered to be transmissible. To convey information, a sequence of changes that can be detected at a receiving point are superimposed on the carrier signal. The changes imposed correspond with the information to be transmitted, and are known in the art as “modulation”. Modulation modes common to the art include frequency modulation (FM), amplitude modulation (AM), quadrature amplitude modulation (QAM), phase-shift keying (PSK), and amplitude-shift keying (ASK).
It is also fully comprehended that as technologies and protocols emerge and evolve for wireless data transmissions, additional interleave, channel coding and modulation schemes may become available.
In cases where a channel is considered stable, a communication system may use permanently assigned interleave, channel coding, and modulation configurations selected from a performance chart maintained in the memory of individual system devices. However, where a channel has significant quality fluctuations, an adaptable interleave and/or channel coding and/or modulation mechanism could be used to select the optimal settings for transceiving a data stream.
For example, signal degradation issues may arise when mobile communications systems (e.g., asymmetric multicasting, broadcasting, etc.) lack the ability to update their interleave, channel coding and modulation configuration. A mobile channel may experience varying types and degrees of signal interference based on its position in relation to the geographic features of its locale. Having the use of only one interleave length or type, channel coding or modulation mode for a mobile radio in rough terrain could result in loss of data or poor channel optimization. Adaptable interleave, channel coding and modulation mechanisms permits the optimization of signal transmissions as a mobile wireless unit traverses through widely varying topographical conditions. Currently, standard methods of error detection and correction in mobile communications systems fail to account for influential factors such as the topographical conditions of the mobile environment. These factors can lead to significant channel control overhead traffic inefficiency and poor data service.
Current mobile communications systems often employ an “acknowledged/not acknowledged” (ACK/NAK) protocol where a receiver detects transmission errors in a message and automatically requests a retransmission from the transmitter. Usually, when the transmitter receives the request, the transmitter retransmits the message until it is either correctly received or the error persists beyond a predetermined number of retransmissions. A separate return channel is often used to transmit the request signal from the receiver back to the transmitter. However, dedicating channel resources and device power to such a trial-and-error based methodology is inefficient and expensive.
Therefore, there is a need to optimize transmission parameters for mobile communication devices in real-time based on terrain and line-of-sight information to overcome the effects of signal blockages and reflections from the surrounding topographical features while minimizing the channel resources required for coordinating such optimizations between multiple devices.
As such, it would be desirable to provide a system and a method for varying the interleave, channel coding and modulation parameters of a system of communications devices based on their relative positions as well as the topographical nature of those positions.